Enhancing the performance of TCP over Wi-Fi power saving mechanisms

Abstract

The Wi-Fi technology is quickly being adopted by new types of devices that pose stringent requirements in terms of energy efficiency. In order to address these requirements the IEEE 802.11 group developed in the recent years several power saving protocols, that are today widely used among devices like smartphones. In this paper we study, by means of analysis and simulation, the effect that these power saving protocols have on the performance/energy trade-off experienced by long lived TCP traffic. Our study unveils that the efficiency of Wi-Fi power saving protocols critically depends on the bottleneck bandwidth experienced by a TCP connection. Based on the obtained insights, we design and evaluate a novel algorithm, BA-TA, which runs in a Wi-Fi station, does not require any modification to existing 802.11 standards, and using only information available at layer two, improves the performance/energy trade off of long lived TCP connections, whilst also exhibiting a notable performance with Web traffic and TCP Streaming.

Keywords

Notes

Acknowledgments

This work is partially supported by the Spanish government through project TEC2010-20527-C02-01.

Appendix

In order to simplify the analysis while capturing the essence of the controller used in BA-TA we abstract the behavior of TCP in the following way. We notice that the RTT experienced by a TCP connection depends on the trigger interval in BA-TA, int(n). Therefore, we assume that between interval updates in BA-TA, a long lived TCP connection delivers a throughput that is inversely proportional to the trigger interval used by BA-TA, i.e. \(thr(n) = \frac{A}{int(n)}, \) where A is assumed to be a constant value in this simplified model (TCP is assumed to converge within a time equal to Tupdate × countmax), and n represents the n-th update interval of BA-TA. Under this premise, the ratio value computed by BA-TA in the n-th interval update can be expressed as:

$$ ratio(n) = \frac{thr(n)}{peak\_rate} = \frac{B}{int(n)} $$

where B is again a constant. Now recall from Eq. 8 that the error incurred by BA-TA in the n-th interval update can be computed in the following way:

Therefore, when \(k \rightarrow \infty\) the selected interval converges as (1 − Gratiomin)k, where 0 < ratiomin < 1. It is then easy to see that int(n + k) has the following convergence properties as \(k \rightarrow \infty: \)